1. Field of the Invention
The present invention relates to a terrestrial telescope with a digital camera that uses a quick-return half-mirror to split the optical path so that the optical path goes to the image pickup device and the observation optical system.
2. Description of the Prior Art
Terrestrial telescopes having a magnification factor ranging from about 20 to 60 are used extensively for observing wild birds and other fauna. Terrestrial telescopes include those based on a Galilean telescope configuration comprising a positive (convex) lens and a negative (concave) lens that functions as an erecting system, and those based on a Keplerian telescope configuration comprising just a positive (convex) lens, to which are added prisms or other such elements to constitute an erecting system. Telescopes thus configured enable a user to observe an erect image.
As well as being able to use such telescopes to observe natural flora and fauna, users want to be able to record the images they are seeing. The prior art includes a configuration comprising a terrestrial telescope to which an imaging mechanism is added to constitute a terrestrial telescope having a digital camera.
Except for the structure of the observation optical system, the structure of the main optical system of the terrestrial telescope with the digital camera is similar to that of a single lens reflex camera. That is, a quick-return (hereinafter “QR”) mirror is inserted into the optical path of the main optical system (which functions as the observation optical system) to guide the image to the observation optical system.
The QR mirror can be a total-reflection type mirror. However, a QR half-mirror is used in the case of a digital imaging system. A spatial image formed by light reflected by the QR half-mirror and passed through an erecting optical system can be viewed by a user via the ocular lens of the observation optical system. In addition, light transmitted by the QR half-mirror falls incident on the image pickup device. By retracting the QR half-mirror from the main optical axis during imaging, the total amount of the light that has been split can be made to fall incident on the image pickup device. The advantage of this configuration is that during the observation period, autofocus control and exposure control can be carried out based on image signals acquired by the image pickup device via the half-mirror.
However, if the QR half-mirror is a parallel plane glass plate, there is a problem of deviation along the optical axis between the position at which an image is formed by light from the optical system transmitted by the QR half-mirror and the position at which the image is formed when the QR half-mirror is retracted from the optical axis. Moreover, shifting of the optical axis also results in a discrepancy between the images obtained by the image pickup device during observation and during imaging, producing differences between the image during autofocus processing and the image that is actually taken. This causes the imaging to be carried out with a focus position and field of view that are different from those the user intended to use. Astigmatism of the image transmitted by the half-mirror disposed at an angle of 45 degrees further arises from the difference in the length of the optical paths in the vertical and horizontal planes. This degrades the sharpness of the image, which has an adverse affect on the autofocus processing.
In JP2002-340426 A1, the present applicant proposed a configuration to correct the axial deviation in image position by inserting a parallel plane glass plate perpendicular to the optical axis, and in JP2003-12953 A1 proposed a configuration in which a parallel plane glass plate is inserted into the optical axis to correct the shift in the image position.
However, these prior-art configurations increase the number of optical components and require a mechanism for retractably inserting the correctional parallel plane glass plate in addition to the QR half-mirror, increasing the complexity of the mechanism and the cost. Also, since the parallel plane glass plate is inserted during the imaging, the amount of light incident on the image pickup device is decreased, resulting in a photographic image that is dark compared to the original optical performance.
An object of the present invention is therefore to provide a terrestrial telescope with a digital camera that uses a simple configuration that enables the image-formation errors caused by the QR half-mirror to be corrected without the insertion of correctional optical components.